Various ophthalmometers that measure the shape of a patient's cornea have provided physicians with important information for over a century. Such devices measure individual variation in average paracentral corneal radius of curvature. However, ophthalmometers do not actually measure the entire corneal topography. Instead, they assume that the central 3 to 4 millimeters of the cornea is a toroidal surface, and measure its "average" radius of curvature in two perpendicular meridians. This assumption is reasonable for normal corneas. Modern ophthalmometers are calibrated to read corneal radius of curvature in "diopters" rather than in millimeters.
Various methods for using ophthalmometers and other devices have been used to measure the cornea. The majority of these treat the cornea as a close approximation to a convex sphere, and require a specularly reflecting surface. These two requirements often preclude the use of such methods intraoperatively or in pathologic cases.
Ophthalmometers, modern versions of which are called keratometers, are indispensable for fitting rigid contact lenses. Keratometers now also are used to estimate the power of intraocular lens (IOL) implants, i.e., to measure the corneal refractive power and then calculate the IOL power required. Keratometers also are used in operating rooms to make corneal measurements which hopefully help the surgeon minimize postoperative astigmatism in cataract surgery by measuring the average astigmatism of the cornea in the center thereof.
A problem with keratometers is that they do not measure the actual corneal topography, but merely the "average" radius. The central region of the cornea, which is the most important optically, is not evaluated. A specularly reflecting surface (i.e., a surface which reflects in accordance with Snell's law) is required, and the measurements are not very reproducible because the cornea is not precisely spherical, so the result of a measurement depends on where the non-spherical cornea measurement is taken. Various improvements in keratometers have been proposed, but have met with limited success because certain fundamental problems, subsequently discussed, have not been solved.
In normal eyes, the interfaces 3A, 3B, 2A, and 2B of the ocular media, as shown in FIG. 1, specularly reflect a small amount of incident light. Since these are optical surfaces, catoptric images (i.e., virtual images) are formed by each of the four interfaces. The magnification of these images depends on the curvature of the reflecting surface. All of the currently used clinical instruments measure the size of images formed by the anterior corneal surface (called the first Purkinje image) which can be used to calculate the radius of curvature.
Typically, keratometers project a luminous object of known dimension onto the cornea, and the size of the reflected image (called the mire) is measured. Alternatively, the object size is varied until an image of specified size is obtained (as in the Javal-Shiotz keratometer). From this information the image magnification, and in turn the corneal curvature, can be determined.
It also should be appreciated that the size and curvature of the cornea require that the illuminated pattern of concentric circular rings be very large (i.e., twelve inches in diameter if it is located well away from the eye to allow access during surgery), or else if it is small (i.e., one inch in diameter), it must be very close to the eye in order to have a virtual image the size of the cornea. This prevents the surgeon from gaining access to the eye while measurements are being made. If the concentric circular pattern is very large (i.e., twelve inches in diameter), then "shadowing" caused by the nose and eyebrow will prevent data from being accurately measured in the shadowed regions.
A major problem in measuring the mire size is that the living eye, and consequently the reflected image, constantly moves. To deal with this problem, a translatable biprism is placed in the viewing telescope, as shown in FIG. 2, to produce two complete mires. The displacement of the images can be altered by changing the separation between the objective and the biprism. As the eye moves, both images translate in tandem. The observer adjusts the biprism so that the mires are tangent, and the reflected image size can be determined by the biprism position. The corneal curvature then can be calculated from the measured size thereof and various optical layout distances of the measurement instrument.
This technique has several inherent disadvantages. First, it does not measure the corneal topography, but merely the average radius of curvature between two points in each of two perpendicular meridians. This is of little value if, as in most pathologic cases, the central cornea bounded by these pints is not regular (spherical or toroidal).
Thus, the translatable biprism technique fails in precisely those instances in which topographic data would be most valuable. Furthermore, the central cornea, the region most important visually, is not measured at all. The lack of topographic data for the central cornea limits the applicability of translatable biprism devices in even routine situations (e.g. contact lens fitting for regular corneas).
A second disadvantage of keratometers is that there is no way to reproducibly align the instrument. Even normal corneas are somewhat aspheric, so each area of the cornea has a different radius of curvature. Thus, measurements cannot be repeated. The translatable biprism keratometer contains a fixation system that limits measurements to the same general vicinity. However the inability to repeatably measure the same spot makes it difficult to follow progressive contour changes, which is valuable in certain diseases. Further, the fixation system cannot be used by patients with poor acuity, which s common in corneal disease. In nonfixating patients (i.e., those who cannot see well enough to fixate on an illuminated target), the machine can be only grossly aligned, severely limiting the keratometer's utility for monitoring progression of corneal disease.
A third disadvantage of translatable biprism keratometers is that they require that the reflected image lie in a plane. If the two test points (on a given meridian of the cornea) are associated with significantly different curvature, they cannot be simultaneously imaged, and a measurement cannot be made. This is frequently the case when the cornea is irregular. Consequently, the translatable biprism keratometer cannot measure rotationally asymmetric corneas, which is problematic because diseased corneas frequently are rotationally asymmetric.
Fourth, the cornea must be specularly reflecting. Epithelial edema, opacities, surface asperities, etc. preclude the use of translatable biprism keratometer techniques. This is especially problematical in an operating room, where even normal corneas frequently are temporarily rendered diffusely reflective by surgical manipulation, i.e., by abrading epithelial cells from the cornea.
The most serious of the foregoing problems is that keratometry does not provide topographic data for the center of the cornea. Nonsphericity in the cornea is indicated by deviations from circularity in the image. For instance, a toric cornea produces elliptically shaped rings, the long axis indicating the largest radius of curvature. Several efforts to overcome this have been devised.
Keratoscopes use targets composed of multiple coaxial circles, usually in a three dimensional arrangement (e.g. cylindrical, spherical, or conical). In order to cover the entire cornea, the target must subtend a large angle at the eye and should not be shadowed by facial features such as the nose, eyelid, or eyebrow. The mathematical analysis of the reflected image depends on the target geometry, and can be significantly simplified by a judicious choice of target architecture. The keratoscope may be fitted with a photographic or video camera, in which case it is called a photokeratoscope. Photographs can be measured and analyzed quantitatively, or video cameras can be directly interfaced to computers which analyze the data received.
To overcome the alignment problems, several autocollimating techniques have been advocated. These systems use the anterior corneal vertex as a reference point for aligning the instrument. In the short term, this improves the measurement reproducibility. However, the greatest need for repeatable measurements is under conditions in which the corneal topography changes over time. In these cases, the vertex position also changes significantly. Since the vertex position used for alignment is not constant, this method is of limited value in such cases. Autocollimating keratoscopes improve the accuracy of keratoscopy for applications such as contact lens fitting. However, they are expensive and difficult to use, and do not improve contact lens fitting sufficiently to overcome their negative qualities. None has been commercially successful.
Neither keratometry (which projects a single ring) nor keratoscopy (which projects many rings) can measure highly irregular corneas because the first Purkinje image cannot be observed for irregular corneas. The image produced by a highly irregular cornea does not lie in a single plane, as shown in FIG. 3. If the axial extent of the image exceeds the depth of focus of the optics used to observe the image, some data will be lost. The problem is often severe enough to prevent a meaningful measurement. Even regular corneas with radii of curvature beyond the limits of the observation optics cannot be measured.
Both keratoscopy and keratometry rely on images formed by specularly reflecting surfaces. Even if the cornea topography is regular, certain medical conditions, such as opacities, inflammation, epithelial edema, abrasions, etc., may render it nonspecular. Keratoscopy and keratometry techniques fail in such cases.
Stereophotogrammetric techniques have been employed to overcome this problem. In this approach, a three dimensional reconstruction of the cornea is made from two preferably simultaneous photographs taken at different angles. To accomplish the three dimensional reconstruction, there must be a number of readily identifiable "landmarks" on the corneal surface. Talcum powder has been used experimentally to provide such "landmarks". A pattern projected on a fluorescein covered cornea also has been proposed as such a fiducial marking. This approach requires the fiducial pattern be on the corneal surface. Fluorescein, however, diffuses throughout the cornea and into the anterior chamber of the eye, making a surface measurement impossible. A soft, flexible contact lens with 200 fiducial marks on its posterior surface has been used with limited success. Stereophotogrammetric techniques cannot be used in the operating room because the fluorescein cannot be localized to the anterior corneal surface, and talcum powder cannot be used in an open surgical procedure in an operating room.